Microsoft have released IE9 Platform Preview 3, an application that gives developers access to the IE9 rendering engine (it's not a full browser). In this update they have added hardware accelerated HTML5 Video, Canvas, Fonts (using WOFF) and big improvements in JavaScript with ES5, DOM Traversal, L2 and L3 events and 83/100 Acid3 score. It sits between Firefox and Chrome 6 on JavaScript speed, but outperforms every browser in real tests.

AFAIK Safari 5 (and WebKit in general) is the most ACID conformant browser. Which, is why - one would imagine - WebKit has been adopted by Google, RIM, and Nokia.

I don't think they care much at all about being ACID compliant. They chose WebKit because it's fast, and therefore well suited to be run on phones and other low power devices, and because the codebase is by far the cleanest, easiest to understand, and easiest to integrate into their own products of the main browser engines.